Technical Abstract:
Determining the interaction and impact of surface runoff and subsurface flow processes on the environment has been hindered by our inability to characterize subsurface soil structures on a watershed scale. Over 40 km of ground-penetrating radar (GPR) data were collected and evaluated in determining subsurface hydrology four small watersheds in Beltsville, Maryland. The watersheds have similar textures, organic matter contents, and yield distributions. Although the surface slope was greater on one of the watersheds, slope alone could not explain why it also had a nitrate runoff flux that was 18 times greater than the other three watersheds. Only a with a knowledge of the subsurface hydrology could the surface runoff differences be explained. The subsurface hydrology was developed by combining GPR and surface topography in a geographic information system framework. Discrete subsurface flow pathways were identified and confirmed with color infrared imagery, real-time soil moisture monitoring, and yield monitoring. The discrete subsurface flow patterns were also useful in understanding observed nitrate levels entering the riparian wet-land and first-order stream. This study demonstrates the impact subsurface stratigraphy can have on surface and subsurface fluxes exiting agricultural lands, even when soil properties, yield distributions, and climate are similar.